I have been running into a few issues with Tubes for my mk47 I have a few of the Rca gold pin 408a tubes and a bunch of the Phillips 6028 tubes . I've noticed the rca tubes I have are a bit noisier but thicker sounding than the Phillips . ( I could be imagining this ) but more strangely the voltage is much higher for the rca tubes with the adjustment pot . The rca tube almost max out with the adjustment pot turned down The Philips have room for adjustment . Anyone notice voltage differences with different Brands of 408/6028 tubes ?